text:
What does the ‡ symbol in the Chart Supplement (A/FD) designate?
In the Chart Supplement there is occasionally a ‡ symbol after the operating hours for control towers (and other time information) like what's shown below.

What does the ‡ symbol represent?

I'm pretty sure the ‡ is what you're talking about - if not let me know & revert my edit :-)

The ‡ symbol in the A/FD specifies that the location observes daylight savings time.
This is from the A/FD legend, which can be found online by going to the online digital A/FD and searching for an airport.  The A/FD Legend link will then be at the top of the search results (unfortunately the link changes every cycle so I can't include an exact link).

6 TIME CONVERSION
Hours of operation of all facilities are expressed
  in Coordinated Universal Time (UTC) and shown as ‘‘Z’’ time. The
  directory indicates the number of hours to be subtracted from UTC to
  obtain local standard time and local daylight saving time UTC–5(–4DT).
  The symbol ‡ indicates that during periods of Daylight Saving Time
  effective hours will be one hour earlier than shown. In those areas
  where daylight saving time is not observed the (–4DT) and ‡ will not
  be shown. Daylight saving time is in effect from 0200 local time the
  second Sunday in March to 0200 local time the first Sunday in
  November. Canada and all U.S. Conterminous States observe daylight
  saving time except Arizona and Puerto Rico, and the Virgin Islands. If
  the state observes daylight saving time and the operating times are
  other than daylight saving times, the operating hours will include the
  dates, times and no ‡ symbol will be shown, i.e., April 15–Aug 31
  0630–1700Z, Sep 1–Apr 14 0600–1700Z.

It is important to note that the period of daylight saving time is different in different countries. In Europe daylight saving time starts 0200L last Sunday in March too, but ends last Sunday in October. And on southern hemisphere it's the other half of the year.

@JanHudec While that is true, it isn't really important here since this is an FAA publication that only covers US airports.  That quote is directly from the legend.  :-)

Or, alternatively, the answer is that the tower has the same (local) operating hours no matter if it's DST or not, i.e., it's always 9-5 local.  However, since all hours are written in Zulu and the conversion to Zulu and local can change depending on if it's DST or not, this provides a quick shortcut to duplicating every entry's times.
More than anything else, this is a cautionary note that one should never speak 'x Standard time' unless you mean it - just say 'x time' instead.


text:
How can I convert an FAA pilot's certificate to a Canadian one?
What do I have to do to obtain a Canadian pilot's license if I already have an FAA one?

Transport Canada has all the information here for each FAA license type (and type ratings); the necessary forms and contacts are here. These are the basic requirements for a private pilot, for example:

6.1 Conditions for Conversion - Private Pilot – Aeroplane.
(1) Must hold an FAA Private airman certificate with airplane category and
  class rating(s).
(1) Age: Must be at least 17 years of age.
  (2)
  Medical fitness: Must hold either a Category 1 or Category 3 TCCA
  medical.
  (3) Knowledge: Must pass the Conversion - Private Pilot
  Licence – Aeroplane, (FAAPA) written examination on air laws and
  communications. Must be able to read, write and communicate in English
  or French.
  (4) Experience: Must provide proof of a minimum of 45
  hours pilot flight time.
  (5) Skill: No additional practical flight
  test is required.

"Read, write and communicate in French." That'll probably catch a lot of Americans out... :/

Is "either English or French" what is intended?

If they meant "English and French," that is what they would have written. I'm surprised at this, however: you could only get away with speaking French in Quebec, where ATC are bilingual. Seems to me English should be required for a national certification.

If anyone came here like me looking for the ability to just rent in Canada once in a while, You can go here and fill out the form. Apparently it only takes 2 days and costs $45 to get the Foreign License Validation Certificate.
You have to actually go to a Transport Canada office, which is a huge inconvenience if you aren't in one of the few Canadian cities that has one, but when you do go it's pretty straight forward, and your license validation lasts for a year.

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text:
Is a massive parachute coming out from an ejectable fuselage technically possible for a commercial aircraft?
And would it save lives?
This video quotes no sources (except the usual "someone") but has gone viral:
https://www.facebook.com/ScienceNaturePage/videos/761924050606522/

The question was on "skeptics". Feel free to remove it, as it does indeed seem a duplicate.

This question, "Is [it] technically possible...", varies from the question of  "Why don't commercial jet aircraft have a parachute escape system?". Though that question is unquestionably related, I would expect to see different answers here.

I am the OP and I think there is no difference whatsoever. The wording is different but my question is greatly answered there. The only reason why I posted it is that on "skeptics" I couldn't find anything similar, and I didn't remember of "Aviation".

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text:
Are there any mandatory announcements that a captain has to make?
Are there any announcements that a captain of an aircraft must make that is reinforced by law or a rule? (This is concerning all countries)

Are you asking about Canadian regulations specifically (your profile says you're there), or are you asking if any country has that requirement? When you ask about regulations, please always tell us which country you're asking about.

Thank you for your suggestion. I have changed the answer. :)

It seems like my last flight on Spirit the pilots never came on the intercom at all. FA's were the only ones that said anything. I think pilot announcements were another $25

Requiring answers for all countries makes this much too broad. Any fully correct answer must cover more than 200 cases.

Do you mean any country (as in, are there any announcements required by regulation in any country to be made specifically by the pilot) or do you really mean all countries (as in, are there any announcements required by regulation in each and every country in the world to be made specifically by the pilot)?

The US does have specific regulations that require the pilot to ensure that passengers are briefed on certain safety issues, although there's no requirement that the pilot must do it personally.
Here are two regulations from the part 91 (all flights) and part 121 (scheduled airline flights) regulations. There may be others.
14 CFR 91.107(a)(2):

No pilot may cause to be moved on the surface, take off, or land a
  U.S.-registered civil aircraft [...] unless the pilot in command of
  that aircraft ensures that each person on board has been notified to
  fasten his or her safety belt and, if installed, his or her shoulder
  harness.

14 CFR 121.571:

Each certificate holder operating a passenger-carrying airplane shall
  insure that all passengers are orally briefed by the appropriate
  crewmember as follows [...]

I guess that other countries have similar regulations, but it would be difficult to list them all, so hopefully this example is helpful for you.

Regarding 14 CFR 91.107(a)(2): A pilot won't need to talk over the intercom for this. He/she can simply turn on the buckle up seat belt sign.

@FallenUser until he gets that plane full of deaf and blind kids headed to the international pinball competition.  Don't know for a fact, but the ADA may have an effect on whether the "just turn on the sign" is sufficient... hence the frequent (dunno if FAA mandatory or ADA requirement to airline or just coincidence and industry "best practice") "ladies and gentlement, the pilot has turned on the seat belt light..." announcement that comes right after.

But if the pinball players are deaf too they won't hear the announcement. And if they're dumb they won't be able to ask the flight attendant :P

ICAO Annex 6 covers the necessary content on briefings for international flights, and most countries have regulations that mirror those rules for flights within that country. The pilot-in-command (what airlines call the captain) is responsible for briefing all passengers on where to find and how to use:

the emergency exits,
the oxygen equipment and life jackets, if they're required for the flight (which they are on airline flights),
the safety belts or harnesses, and
passenger briefing cards, if carried;

and for ensuring that all passengers are familiar with the location and use of emergency equipment carried for collective use, for example life rafts or escape slides.
That's all that's required worldwide. Although it's the PIC's responsibility, there's nothing wrong with them delegating to other crew, and this is common practice on airline flights.
The airline will also have a standard operating procedure (SOP), and this may specify additional things the PIC has to say to passengers. Although the SOP isn't a law or regulation, if there's an accident in the flight, and it is found to have been caused by the PIC not following the SOP, the PIC will likely be blamed and this will probably cause them legal problems afterwards.

Is the PIC really responsible for briefing the passengers, or are they merely responsible for ensuring that the passengers are briefed? That's a big difference, and in the context of this question, I think it matters. See e.g. Pondlife's quote from US 14 CFR 91.107(a)(2), which is about PIC ensuring people onboard have been briefed, as opposed to specifically the PIC briefing them.

PIC isn't the same as "captain".  If the captain gets up to go to the toilet during a long flight, he's still the captain - but he's not the PIC (or at least, he shouldn't be!)

@MichaelKjörling The PIC is responsible for flying the aircraft too, but that doesn't mean they are always the PF. I don't think the regulations actually make the distinction you're trying to draw.

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text:
Does an airplane need a key to start?
Does an airplane require a key to unlock the doors and start the aircraft engines?
Is it somewhat like how a car does?

My 1973 Cessna Cardinal has a key to unlock the doors and another to start the engine.  Just like a car.
I could use a horn to scare deer & geese off the runway.

I could use a horn to scare deer & geese off the runway :)

Same for '66 Piper Cherokee, and all the other non-glider airplanes I've flown.  And it's not really the deer on the runway when I want to take off that bother me, it's the ones that lurk on the side until you're halfway down, then bolt across :-)

8-point buck got hit by a low wing Piper at my airport late Sunday doing that. Dented the wing lower leading edge, tore the Pitot rube from its mount.  Killed the deer.

On further thought, it's not "just like" a car.  In a car (at least before the newer push a button ones), you turn the key to start.  In the Cherokee and IIRC other planes, the key controls the magnetos, so it has 4 positions: off, left, right, and both.  To start, you turn the key to "both", and press a button to engage the starter.  Then when you do your runup, you switch to "left" and "right", and check for an RPM drop (and that the engine keeps running!).

In older Cessnas (early 70s, what I am familiar with), it is like a car. Turn full clockwise to spin the engine & engage the magnetos, release to both, then counterclockwise one position to one mag, two positions for the other mag.  There is no separate start button.

@CrossRoads: Yes, but there's still the 3 positions for the magnetos (and no ACC position :-)), so not "just like" a car.

But it's like a car in a lot of ways.

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text:
What is the EASA equivalent of the FAA's 91.517?
What is the EASA equivalent of the US 91.517 regulations (on passenger information)?


text:
Are there any airplanes with the Centre of Gravity behind the Centre of Pressure?
We are used to seeing a forward CG in all airplanes, from a small Cessna to a large Boeing.
I wonder whether any airplane with CG behind wing CP exists in production? Is it even possible? If so, what about its stability?
EDIT: I mean wings CP, because it is the most common definition of the airplane CP. Usually for civil aircrafts the wings produce lift, and the tailplane produces downforce to counteract wings rotation moment about CG. These aerofoils (wings and a tailplane) have 2 separate CPs and two forces in the opposite directions. I ask about the wings CP in normal wing AoA ranges (excluding aerobatics, critical AoAs etc.)

In steady flight both points are exactly at the same lengthwise location. The pilot is able to shift the center of pressure somewhat by moving the elevator. Therefore, every aircraft that pitches up will become one where the center of gravity is behind the center of pressure.
But I guess you did not request this answer but a different one. That, however, would require a different question: Are there any airplanes with the Centre of Gravity behind the Aerodynamic Centre? Please read here about what those different centres are.
Most aircraft are indeed built such that the center of gravity is ahead of the aerodynamic center. This allows the pilot to let go of the stick momentarily and gives him valuable force feedback on the stick when the aircraft's speed or load factor changes. And it is required for civilian aircraft so they can be certified.
However, placing the center of gravity further aft has become a de facto standard for military combat aircraft with supersonic capabilities. Please read here why this helps. To summarize:

Agility is enhanced
Supersonic trim drag is greatly reduced
Smaller wing and tail surfaces are needed for the same take-off and landing performance.

The first aircraft which was designed with the center of gravity behind the aerodynamic center was the Wright Flyer I. When designers found out that aircraft could be made naturally stable, they placed the center of gravity more forward, and only with fly-by-wire controls did they return to the rear center of gravity location. The first design to do so on purpose was the F-16, and today every fly-by-wire combat aircraft with supersonic capabilities follows the same path.

+1'd but I feel your last paragraph disagrees with your source, which states that longitudinal stability was well understood by the time of the Wrights and that "[...] they experimented successfully with an aft tail. They knew that the configuration could easily be made stable. [...] the canard configuration, the most distinctive feature of the Wright aircraft, was not based on sound technical grounds of stability. It was rather a matter of control in pitch, especially under extreme conditions." From your answer I got the idea that rear tail stability was developed later. And ty for the read

@AEhere: The first aircraft designers like Penaud and Lilienthal copied birds. Maybe they understood stability already; from what I read of the Wrights they clearly did not. They noticed the restlessness in pitch and decided to move the cg aft. That this helped at all was caused by the increase in pitch inertia which lowered the eigenfrequency and made the aircraft easier to control. I have read a book from 1911 where the author clearly was clueless about pitch stability. My impression really is that it took until around 1915 for pitch stability to be widely understood correctly.

CG and CP are usually NOT at the same datum. It is also a stability issue, as I understand, and that's why almost all civil aircrafts have a tailplane, which creates downforce and counteracts rotation mentioned in your first link. An aircraft pitches up (or down) when sum of the wings and the tailplane moments are not zero, and it does not mean that CG is behind CP. Elevator movement mainly changes a tailplane angle of attack (and sometimes the tailplane camber), the airplane rotates, lift increases due to wings AoA, and rotation stops. But thank you for the fighters links.

Aerodynamic centre is not I was interested in, because it is just an empirical point: no any force acts there. But I agree, the question wording is not perfect. Probably I should ask about CP location in allowed AoA range.

@avtomaton: The wing has an CP, and so has the whole aircraft, including the tail. The latter one is the one I am talking about, and it should be obvious that it must be at the same datum as the CG for stationary flight.

@PeterKämpf you mean inertia in a generalized sense, right, (also called pitch stiffness in some places)? As opposed to the actual $I_{yy}$ .

@PeterKämpf I am sorry, but what is 'CP of the whole aircraft'? It does not make any sense! Usually in aerodynamics CP of aerofoil is considered, and wings and a tailplane are considered separately. When we are talking about CP, we assume that we consider a sum of lift forces field, and find a point as a centre of superposition. When we have different aerofoils (like a wing and a tailplane), we usually cannot consider the lift forces as one field because we usually want to consider also their moments. And when we talk about CP, we mean wings CP (and aerodynamic centre as well).

@avtomaton: Then the CP of a wing should also not make sense to you. In all cases it is the point where all vertical pressure components can be summed up. You cannot look at the parts separately and add them. They interfere with each other, so they must be looked at in combination. If you think of a CP as something only a wing can have, consider this a chance to improve your understanding.

@PeterKämpf I refer to conventional training books (ATPL books from Jeppesen, Oxford and Bristol), aircraft aerodynamics books and some internet resources like skybrary, avstop.com, flightlearnings.com. Most of them consider CP and Aerodynamic centre of the aerofoil like a wing and a tailplane, and barely consider CP or aerodynamic centre of the whole aircraft. Probably you can share some references to me? I would appreciate more resources for further reading. Actually your answer is the first source where I see CP of the whole aircraft consideration instead of wings and a tailplane.

@avtomaton This is highschool physics, the airplane needs to be in equilibrium, so the combined forces on all the bodies (wings, tail, fuselage, etc) need to balance out. The step from the individual wing CP to the CP of the whole airplane is seldom taken in the books because it is assumed to be a trivial conclusion, and most of the time it is easier to forget the fuselage exists and just add up weight + wing lift + elevator lift.

@AEhere OK, I updated my question. It is obvious that the whole airplane CP should be always at the CG datum at all airplane AoAs, when the airplane is stable. I don't see any reason that ask about those trivial things. Stability of the airplane is usually determined from CG, wings CP locations for different AoAs and a tailplane CP location for different AoAs, isn't it? Now I understand that I just created a poorly worded question, I tried to update it.

The reason for a CG forward of the CP is that this configuration is generally longitudinally stable, similar to an arrow or dart. However, highly maneuverable aircraft, such as military aircraft, might have a CG behind the CP which makes the aircraft longitudinally unstable but more controllable. Complex computer controls will maintain controllability of the aircraft even though it is unstable.
https://en.wikipedia.org/wiki/Longitudinal_static_stability
checkout static margin and the diagrams in the above link.

I understand the theory, but I asked about some examples... I understand that in theory that kind of airplane would demonstrate high instability, but I wonder whether they are really manufactured and used.

sorry, I've been looking for a while and cannot find any specifics, besides some balancing examples in army manuals. My guess would be that supermaneuverable aircraft such as the f-22 have a CG envelope that goes behind the CP.


text:
What data file format is used by FDRs?
For analysing the FDR, data is downloaded to the local computer from the FDR and feed into the Analysing Software. I would like to know what is the file extension of the data stored on the FDR.
I came to the conclusion that it must be csv for storing all the data about Height, longitude, latitude and so on. Then this .csv file is feed into the Analysing Software for analysing. But I am not sure if I am correct.

it's a custom proprietary file, it's more likely is that it's a densely packed binary file than that they included text processing in a safety critical application.

To expand on what ratchet is saying, the FDR is not a "Windows" or probably even Linux based operating system. It doesn't take the feeds from the sensor inputs and save it to a standard human readable file. The data is probably stored in circular non-volatile buffers on the FDR and a proprietary software is used to read the buffered data out. The software that reads it then places it in some sort of Windows/Linux file format which I'm sure can be converted to something human readable. Its certainly not stored on the FDR that way though.

You might want to rephrase this to ask "what data file format is used by FDRs?" or something similar. You seem to be assuming that the files are in a 'typical' delimited text format, but it looks like it's a lot more complicated than that.

@RonBeyer Warning!! the following question is stupid: If I am writing my own FDR analysing software then I could just write the Software in such a way that the feed data is in .csv format. Probably those company who makes FDRs has the software to convert the FDRs data into .csv format.

You could, yes, but if you really sat down and thought about how the data comes in and how you have to only keep X minutes worth of data in a finite storage, CSV becomes a lot less practical. When working with embedded systems its a lot easier to work with the memory directly than to try to make a human readable file format that doesn't need to be human readable. Its not like they plug an FDR into a computer and it shows up as a flash-drive.

I see VtCs, but I'm not sure this is unclear or too broad. The OP is making some assumptions that we all seem to feel are incorrect, but the comments/answers should serve to correct those misconceptions. Unless different FDR manufacturers use different data formats (entirely possible), I wouldn't think it's too broad. If that's the case it could be narrowed easily by specifying the manufacturer(s) he's interested in. /my 2¢ worth

CSV is an incredibly memory-inefficient and ill-defined format, and it is extremely unlikely that data is stored like that.

The builders of an FDR must comply with DO-178C (the older DO-178B may also be of interest here) as per AC 20-141B issued by the FAA. These documents do not specifically define a data format but provide requirements for what it must be resilient against. Here is an interesting research paper that covers some of the topics you are asking about. Since most avionics use ARNIC 429 as their standard of communication some FDRs provide ARNIC 429 inputs to record the avionics instructions at the time. I would assume these inputs dump the bus data right to the storage medium (most likely solid state memory these days). Since the formats the device uses may be proprietary as well as very low level (and memory efficient) I am sure they make software to dump it to other formats. For example L3 makes a full unit for their FDRs.
After some more searching I found what may be the most comprehensive answer from this Boeing summary document

What Is a Data Frame?
A flight data recorder (FDR) data frame is the
  order of the words that are transmitted from the digital flight data
  acquisition unit (DFDAU) to the digital FDR (DFDR) each second over
  many seconds (see ARINC 717 for additional information). Most FDR
  system data frames are made up of four subframes within one
  superframe. For a 64-words-per-second (wps) FDR system, a DFDAU will
  output 64 12-bit words to the FDR each second, where each word
  typically contains the value of an analog parameter. The order of the
  words (for example, word number 12 of the 64 words) within a subframe,
  as well as the order of the subframes, define an FDR data frame. This
  order is important to understand in order to decode the data recorded
  in the DFDR.

It should also be noted although they may have all been phased out by now, some older cockpit voice recorders and FDRs recorded on analog mediums and may still be inservice doing so.

The link 'L3 makes a full unit for their FDRs' is broken.

We can use inductive reasoning to make a guess at a probable answer to your question.
Your premise:

I came to the conclusion that it must be csv

is likely wrong.
The only purpose of a CSV file is to transfer data between a source and a sink which do not understand the way in which the other stores data.
An FDR does not need to be understood by any other sink other than the manufacturer and/or incident investigators.
Adding the ability to store or retrieve data in a CSV format would add some complexity, through additional problems to solve, which is simply not needed.
All that is required is a method to extract the raw data and display it in a format that humans can understand.  This is trivially done with a piece of custom software.
The designers will have no requirement to provide data in any format other than that required by the extraction software so they simply won't implement it.
CSV, and other data interchange format files, all have limitations.  They lose resolution, they are inefficient in their use of resources and only exist to work around problems introduced by a lack of standards, differing interpretations of standards and attempts by commercial, general purpose software vendors to differentiate themselves or make it more difficult for systems to interoperate.
We can therefore reasonably conclude that an FDR does not store data in a CSV file.  Of course, the custom software which does extract the data might contain such a feature but then again, why would it?

Of course, the custom software which does extract the data might contain such a feature but then again, why would it? because once you extract the data you want to save several copies and a format easily readable by most programs offers the lowest possible barrier to the advance of the investigation?

@Federico Why? What requirement is there to do so? Even if we accept that there is such a requirement, there are more effective common file standards for the kind of data an FDR stores such as tab delimited, JSON etc. but they all have significant limitations.  If you asked me to design this, I would simply use a proprietary, raw data format and make the consumer software easily available to those who need it - with graphical plots, flight simulator feeds etc.  I wouldn't care about 3rd parties since they are not the consumers of the data.

I can't imagine other than early in an investigation the data from the FDR will be converted to a variety of formats, including csv (for easy loading into spreadsheets), whitespace-separated unixy files (for ad-hoc analysis with a scripting language), and possibly others. Which of these particular formats comes out of the vendor's reading step is not really important; it is easy enough to interconvert between them.

Something additional: Critical data contain a mean of integrity verification (CRC-like). The storage space will be managed in binary form for speed and space efficiency, not in a human readable form for the reasons explained (and a boolean flag won't waste a byte). It is not sure data will be linearly stored, as EEPROM cells are limited to a certain number of writing cycles, so the need to optimize where a byte can be stored That should be more similar to chained allocated buffers (malloc-like) than to file records.

Simon, I've answered you in chat, if you would like to pass by: http://chat.stackexchange.com/transcript/message/28471647#28471647
Also, have a look at the question linked by Shalvenay just below.


text:
Are tower controllers trained on not overloading pilots with complex taxi instructions while landing?
The above question comes from the following observations:
I've noticed over the years of flying (small GA aircraft) that some towers are talking more and more to me while landing. It used to be something like

Cessna 123, exit Charlie and contact ground.

Which is fine, but sometimes I get not just exit instructions, but full, complex taxi instructions while I'm still in landing rollout. The other day, as I was landing, I got the following:

Cessna 123 where are you parking?

I tell them, and they come back with,

Cessna 123, exit taxiway Juliet, taxi to parking via Bravo, Golf, Foxtrot, and cross runway 27 at Bravo, with me.

This may be an easy instruction when you are sitting at a desk reading it, but while landing at a somewhat unfamiliar airport, it’s a lot, and I would normally write that down, but not while I'm landing or even on rollout. So, I replied

Cessna 123, Exit Juliet, standby

There are some towers that never do this. I wonder if this is something they do with larger aircraft with two crew members, and when I'm flying into these larger airports as a single pilot they just treat me the same.
I thought the general rule for ATC was not to talk to pilots unless it was urgent during the takeoff and landing. Given all of the recent runway and taxi incidents, I tried to find what guidance ATC has on communicating with pilots (especially single-pilot operators) during those critical phases of flight.

When you say "while landing" do you mean "during the approach or flare" or "after touchdown but before runway exit"?

I really like your "exit(ing) Juliet, standby" approach. It confirms to them the minimum info they'll want: that you will in fact exit where they want you to, while deferring the rest until you're ready. And never be afraid to ask for progressive taxi instructions. ATC gets paid for it and they're not keeping score like in an aircraft carrier squadron.

@DJClayworth "sometimes I get not just exit instructions, but full, complex taxi instructions while I'm still in landing rollout" seems to answer your question.

@DJClayworth its during the rollout when I'm applying back pressure and breaking and my hands and attention is on slowing down and exiting to be followed by a brief after landing checklist. FAA advises not to perform after-landing (or any checklists) while taxiing, so I need about 30 sec or so after exiting runway to run the after landing checklist. If there is a second crew member this can be done while taxiing without any problems.

ATC are specifically advised not to give taxi instructions before touchdown or immediately after. From FAA Order 7110.65Z, section 3-10-9:

Runway exiting or taxi instructions should not normally be
issued to an aircraft prior to, or immediately after,
touchdown.

Please respond with "Possible ATC deviation, advise when ready to copy a phone number."
That is a joke. Responding with "standby" seems like a perfectly reasonable response to ATC issuing irrelevant instructions during a critical part of flight.

Thank you for providing the source!  LOL at "Possible ATC deviation...!"

It's the controller trying to save you the trouble of having to stop on the taxiway and call ground (it's likely the same person if it's a smaller GA airport anyway, so they're saving themselves some hassle as well) and they're assuming you can absorb the info.
They aren't supposed to give instructions until the end of the rollout, so you should be getting it just as you are approaching the taxiway turnoff. But in any case, if you are preoccupied, just say "standby", pull off and stop past the taxiway threshold line, and call back for the controller to repeat. Can't go wrong that way.

I would imagine for pilots familiar with the airport, they know where their usual GA parking spot is already, so the taxi instructions are typically nothing more than a reminder for what they have memorised (and to listen out for any deviation because a taxiway is closed or similar).

Yes, controllers are in fact "trained" not to give complex taxi instructions during rollout.
However, this training seems to be more after-the-fact problem-fixing than an inherent part of on-the-job training. By that I mean that this is an issue which is often reported by pilots; pilots file safety reports and eventually controllers are reminded "don't confuse the pilot" via a printed Safety Bulletin posted at the sign-in desk or perhaps as part of the "read this when you have a moment" quarterly refresher training.
Actual techniques demonstrated and practiced during on-the-job training will depend on the specific trainer. Some trainers, particularly if they are also pilots, will harp on this issue; some will not.

Thank you for this insight, I started to do a safety report, but didn't finish it, but now I'll probably do one especially if it brings attention to the issue.


text:
What is the hole located below the 2nd or 3rd window of Boeing 737?
I see there is a hole located below the 2nd or 3rd window. What is its purpose?

(Sources: flickr.com and planespotters.net)

Thanks for editing! It's a kind of duplicate, but I haven't managed to find that answer and even if I had found it, I wouldn't have been able to match it with my question


text:
What is the meaning of the letters on the side(s) of WWII British aircraft?
WWII British Spitfire and Hurricane had identification letters on the side of fuselage. Do they have a specific meaning? How are the letters chosen?

Spitfires had a few different markings, but primarily there were two. One, written in smaller, often black, text was the aircraft serial number and stayed with it for its entire life. The other, usually either side of the roundel was the squadron and airrcraft id:

Image source
XT was 603 Squadron. So if a Pilot of that squadron was told to go and "Fly Delta" he would know to go to that specific aircraft. Aircraft would have been lettered in sequence, Alpha, Bravo, Charlie etc.
There were some exceptions; to wit

...Thus the famous Douglas Bader was allowed to have the letters "D-B" painted on the side of his Spitfire VA, because he commanded the Wing flying from Tangmere Aerodrome. Bob Stanford-Tuck, commander of the Biggin Hill Wing had a Spitfire with "RS-T" on it.
  source

Hurricanes followed the same standard


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At what Mach Number do variable cycle engines (VCEs) start showing benefits over turbofans?
Currently, most supersonic engines engines are low bypass turbofans. However turbofans start becoming infeasible as the mach number increases. Variable Cycle Engines, still have noise and weight issues but are purported to give significant improvements in spillage, airflow matching and mixed mission cycle efficiency.
I wanted to know what is the lowest mach number (roughly) at which the benefits of the VCE's attributes are purported to give it an advantage over turbofans.
I'm mostly interested in benefits in specific fuel consumption thrust, spillage and matching losses etc. and am not that worried about the noise, weight and complexity problems (though useful information regarding this is appreciated).
most literature regarding this that I found were from the 1960's to the 80's so links to newer resources, literature, etc. are appreciated

It's not a high Mach number that gives a variable cycle engine an advantage over a turbofan. Most air-to-air combat happens at subsonic speeds, because drag gets very high when you start trying to turn sharply at supersonic speeds. For combat, you need high specific excess power (SEP), which comes from a low bypass ratio turbojet. But, cruise speeds are also typically 0.8 to 0.9 Mach, even for fighter jets. But lower fuel consumption in cruise occurs with a high bypass ratio turbofan. That's because the exit velocity is not much higher than the aircraft velocity, which means the propulsive efficiency is high. But, it means the aircraft doesn't have excess power, to suddenly accelerate, or stop energy bleeding off in multiple, rapid turns.
So, in both cases, cruise or combat, speeds are subsonic, but the desirable type of engine is quite different. To illustrate, the specific fuel consumption (SFC) of a CFM56-7 is about 0.37 lb/hr/lb, with a BPR of about 5.4. In comparison, the F100-PW-229 has an SFC of about 0.73 lb/hr/lb (non AB), and a BPR of about 0.36. That's pretty close to a factor of two. Obviously, a VCE engine can't change the bypass ratio as much as in this example. I don't expect the variation of bypass ratio (BPR) in a variable cycle engine is published anywhere.
So, the advantage of a VCE occurs even if you don't need or want to fly supersonic. The advantage occurs when you suddenly need excess power, even at subsonic speeds. Military aircraft benefit from a variable cycle (variable BPR) not because of the large speed range over which they fly, but because of the wide variety of mission types they fly. Civil aircraft just need to be optimised for cruise. Variable cycle allows military aircraft to be better optimised for combat and cruise, not just the first.


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Why turn mobile phones off on take off and landing?
Why do commercial airlines insist on turning phones to airplane mode? Does it really interfere with the planes dedicated frequencies?

Sorry, I'm new to this but I'm not sure the two questions above are the same, else my question would have pointed that out?

It is unlikely that phones and other personal electronics could interfere in a way that would significantly impact communications, navigation or other systems. But, the possibility is still there, and that possibility forces the FAA to impose regulation. In most cases, device-use regulations are passed down to the airlines, and it is the airline that sets rules for device use on their flight. It's quite possible they limit device use during critical flight phases like takeoff and landing to help ensure people pay attention to safety briefings.


text:
Need some help with a seemingly simple question
An aircraft flies from A to B, distance 155 nm, in 1 hour and 23 minutes. If there is a tailwind of 12 kts the TAS is?

Are you unable to calculate the ground speed from that information?  Basic algebra.

How would you calculate the speed of a vehicle from time and distance information when there is no wind?

The TAS is not affected by the wind. The aircraft flies within the mass of air, and the TAS is the speed of the aircraft with respect to that mass of air. The only speed affected by the wind is the groundspeed...


text:
Is there any emergency situation in which jet engines would not be shut down before passenger evacuation of a transport aircraft?
A comment to this answer discusses the possibility of evacuating passengers risking ingestion into the engines if evacuating over the leading edge of the wing. That particular question discusses the Boeing 737. For example:

"I always thought that the reason to go off the back of the wing was to avoid getting sucked into an engine."

and

"I'd imagine on a land landing, the leading edge could bring a passenger dangerously close to being ingested, should the engines be rotating.."

However, I really can't see any valid situation that calls for the following sequence of events:

Emergency landing (either on land or on water)
Keep engines running
Begin evacuation of aircraft
Shut off engines (during or after evacuation)

Instead, the sequence of events would seem to be:

Emergency landing (either on land or on water)
Shut off engines
Begin evacuation of aircraft

in which case some unlucky individual might possibly hit (come into contact with) some external or exposed part of the engine or engine pylon, but I have a really hard time seeing how anything could be ingested into the engine at that point.
For the purposes of this question, I suppose "engine running" can be interpreted as "sufficient intake airflow for large foreign object ingestion to be a concern".
Is there any valid situation where post-emergency-landing evacuation would begin while the engines are still running? If there is, then please describe at least one situation which would call for that and describe why such a course of action would be necessary in that situation as opposed to waiting for the engine front intake fans to spin down.

If the airplane landed in water, the engines would automatically stop running because water puts out the combustion going on inside the engine.

Wild guess: if an engine on one side of the aircraft don't stop, you should evacuate by the other side.

Pure speculation: for any landing in which the pilots are incapacitated or killed, the cabin crew will commence evacuation, but have no access to the cockpit to be able to shut down engines.

I wonder if ingestion of humans is even a major concern, in terms of probability, at idle?

@RyanMortensen maybe you should ask that as an actual question?

@RyanMortensen Ingestion of an adult human - probably not. Ingestion of items being carried by humans - more likely, and that scenario might also turn an "mostly harmless" idling engine into an uncontained fuel fire, bearing in mind that you are only in this situation because you can't shut down the engine normally! Note that the a common way for debris to be ingested is because of a vortex formed between the engine intake and the ground below it. At full engine power, those vortices are quite capable of lifting something the size of a 1-foot cube of concrete from the ground into the engine!

@alephzero - I asked follow-up question about ingesting concrete: https://aviation.stackexchange.com/questions/47169/can-jet-engine-ingest-suck-in-concrete-from-runway

It is possible for the engine to not respond to shutdown sequence, so you'd have to get the passengers off before dealing with the rogue engine.
In the Qantas Flight 32 one engine had an un-contained turbine failure in engine No. 2 leaving them unable to shutdown engine No. 1. This was due to a piece of debris severing all communication to the engine, essentially leaving it out of control and running.
This in turn made it impossible for the crew to activate any safety mechanisms for the engine. It was eventually shutdown by dousing it until flameout.
Report by Australian Transport Safety Bureau
Wikipedia Article on Qantas Flight 32
It should be noted that in this specific case they didn't evacuate immediately due to a fuel leakage and the overheated brakes, though when they did evacuate engine No. 1 was still running.
Another situation where immediate evacuation may be preferable is if the aircraft had a cabin fire, meaning the highest priority would be cabin evacuation, which could potentially happen simultaneously to the crew attempting engine shutdown.

"It is possible for the engine to not respond to shutdown sequence" Good point, but why not pull the fire handle in that case which shuts off fuel to the engine? Sure, it's a somewhat unclean shutdown, but if the engine is not responding to a shutdown sequence in the first place, then surely it's going to have to be looked through thoroughly anyway. I doubt any pilot in their right mind will take off with an aircraft where the log just says "engine didn't shut down on command, nothing was done to investigate the problem or fix it, but trust us, it's fine now"...

@MichaelKjörling in the Flight 32 case all contact to the engine was lost. All the cables were severed when No. 2 engine failed so even the safety systems couldn't be triggered.

@MichaelKjörling I've added that info. I also found the Australian Transport Safety Bureau report that states the evacuation took place with the engines running and the methods they attempted to use to shut the engine down. Have a look at page 8 onwards. It's a good read if you like the nerdy stuff like me :)

Possibly dumb question: Could wind spin a turbine fast enough to ingest a human?

@called2voyage If the wind alone powered the engine (not saying that's possible) then the engine wouldn't actually be sucking air in. So the only thing that would cause you to go towards the engine would be the wind. So really, it's "Could the wind blow you into an engine?"

@Robert Fair point, but the wind has blown around humans before--it would be an exceptional situation, though.

@called2voyage I'm not saying it is or isn't possible (I'd need to think/read up on a few things to answer properly). Just that you couldn't be sucked in in such a scenario.

@called2voyage That would require hurricane force winds. Also, the engine would just be windmilling, so it wouldn't be spinning very fast and wouldn't be creating any suction at all. It wouldn't be significantly worse than being blown into any other large metallic object by the same wind. At that point, you should probably be more worried about this happening.

@reirab: well, no worse than being blown into any other large metallic object that is rotating in a confined space ;-) Basically you're being pushed by the wind, into something that's acting like a wind turbine. But of course if the wind was blowing in the opposite direction you'd be about as likely to be pushed into the other side of the engine with the exits where they are, so take your pick. I guess the back is slightly less worse, but ofc this isn't the scenario that dictates the decision.

@SteveJessop With it windmilling, I don't think the rotation would be fast enough to make the situation much worse than the blunt force trama would be without the windmilling. Either way, though, it's not something I want to try.

I would have thought you would have shut it down by closing the fuel valves. Not the fastest way but it would bring it down.

@Joshua do you honestly think two professional pilots and everyone else involved didn't think of and try that?

@Notts90: I find it more plausible they didn't think of tank select valves as engine cutoff valves than the distributed failure required to leave no working tank select valves between the engine and the closest wing tank.


text:
Are there any helicopter blades that fold in the middle?
Let's assume that, for some arbitrary reason, helicopter blades are too long for transport or storage. Folding them at the rotor hub, as is done with conventional folding helicopter blades, would still be too long and there is a need to fold them further. So, they are folded somewhere in the middle of the blade instead (or as well).
Do such folding helicopter blades exist? If not, has there been theoretical studies on how it could be done? What would be the impact on blade performance?
This is a question about the theoretical feasibility and difficulty of using such folding blades, and whether there have been existing studies or designs for it. As such, the practical applications and whether it would be worth it, or whether there would actually be a real-life scenario where this would be useful, should mostly be ignored.
(Bonus point if it is possible to deploy the folded blades simply by starting the engine, but this is probably too outrageously complicated to pull off anyway.)

Don't think it is a problem, blades can already be folded on several helicopters during transports.

@Bentoy13 This is what I meant by "conventional folding helicopter blades": folding it in one place near the axis. I have never heard of helicopter blades folding mid-section, though, which is what the question is about.

Actually, deploying the blades by inertial force alone is trivially easy. It's making  their hinges and locks strong enough to withstand flight that would be complicated.

@AEhere Indeed, this is what I meant by that: how to have deploying blades by inertial force that can actually do their job as helicopter blades...

I don't think this has been done. The problem with longer blades is that you have to keep the tips sub-sonic. This limits the rotor RPM as the blades get longer. There is a trade-off between blade length and the effectiveness of the blades. At some point it is easier to make the blades thicker or wider than it is longer. For high-load applications, adding blades is more effective. Take a look at Sikorski's new CH-53K blades for comparison.

Even large helos like CH-46's and -53's can fold the blades so that they do not extend past the length of the fuselage. Doesn't seem like a great idea to me to fold them any further.

@RonBeyer That's what I feared: no-one has tried because there hasn't been case where it would be useful enough to justify the cost of developing it yet. Is there no way to have a rough idea about what it would entail without that?

Bending a helicopter blade in the middle is of course possible, but poses many serious problems and risks. The designers would look for different solutions first, like they did for the UH-60.
The blade loads are lift, centrifugal force, and twist. Manufacture the blade with modern composite technology, mount it with the fully articulated hinges that are usually a requirement anyway, and trust the blades with your life because a breakage would be catastrophic.
Any repeated action poses additional risk. Mounting a hinge and lock that operates every time, is not open to human error, and never fails - that is would be the quest for the folding blade designer.
Of course a folding blade can be designed, like the folding wings on navy fighter aircraft. But not for some arbitrary reason. Only for a vital reason that cannot be solved in any other way.


text:
Why are overwing engines so rare?
This comment to a question about underwing vs rear-mounted engines notes the existence of a rarely used third option: overwing engines.
There seems to be very few planes using those. What are the disadvantages that makes it so unpopular, and what are the advantages that made it worth building those few planes?

Wikipedia lists 14 civilian aircraft with overwing engine designs. https://en.wikipedia.org/wiki/Category:Engine-over-wing_aircraft

Related, maybe a dupe?

The biggest reason is the negative (or spoiling) effect of having a body "in the way" and disrupting the flow field above the wing.  A wing makes lift by redirecting a very large package of air downward (actually, up a bit, then down a lot) as the wing moves along, and most of the package of redirected air (which ends up as downwash) is above the wing.  (Otherwise, the wing would just be an air deflector and a flat sheet of plywood would do the job.)  Disruptions to this upper flow have to be minimized.
Look at any tactical fighter, like a Skyraider, festooned with rockets and bombs and tanks under the wing almost across the entire span. The disruptive effects of all those shapes on the underside have minimal effect other than parasitic drag. Put them on top and they act like spoilers and the airplane would never get off the ground.
On airplanes that do put the engines on top, either the engine is so far aft that it minimizes the effect and it might was well be mounted on tail pylons (like the Honda Jet), or the engines are mounted way forward to minimize the effect, plus gain from having the high velocity fan stream across the top, or it's propeller driven and the propeller flow field mitigates the effect, or, it was done anyway and the design was unsuccessful.

It's interesting that the English Electric Lighting sometimes carried some limited amount of overwing stores.

Related answer on question fuel tanks https://aviation.stackexchange.com/questions/47277/have-any-operational-aircraft-ever-used-over-wing-fuel-tanks -- I guess one of th answers is yours

Note however that these kinds of off the wall ways of doing things remain "off the wall".  If it was practical, it would be more widely used.  You see that all over the place in aviation.

At high angles of attack it would be beneficial because the nacelle would no longer be in the way of the inflow as it is with under wing engines. That's why they all have these vortex generators on the engine nacelles, it can stabilize the airflow above the wing which would have been distored by the nacelles in front and forward of the wing - where the air comes from.

The main advantages are keeping the engines clear of ground debris to avoid FOD and providing additional lift for STOL operations, as can be seen in the An-72.
These requirements are rare for civilian airliners operating out of modern airports, and are more common for military transport types and regional jets operating in remote areas.
The greatest disadvantage is the complicated maintenance, which requires stairs to even begin to inspect the engine. Furthermore, with the proliferation of composite structures in aviation, it is desirable to avoid the risk of a mechanic dropping a wrench on such a surface, and having technicians regularly working over the wing is a surefire way to end up replacing panels due to tool damage.

and makes it impossible to remove an engine by putting some blocks under it and unscrewing the bolts, you need a crane and gantry to lift it away.

Another reason overwing engines are not favored for passenger planes is that instead of shielding the passengers from engine noise by the wing (as in the case of underslung engines) you have the noise source(s) on the same side of the wing as the passengers, which makes for a noisier cabin.
In addition, suspending an engine on a pylon under the wing allows the engine to be shed off by gravity in case of an inextinguishable fire that destroys the engine. It's not clear you could exploit this effect with engines mounted above the upper surface of the wing.

The ability to shed the engine is meant to keep the engine and pylon from ripping a hole in the wing's fuel tanks if it comes off during a crash or hard landing.  It has never been meant to be used in flight (although that does occasionally happen, typically resulting in bad things happening).

The reason why engines are mounted under the wing is easy maintenance access and easy visual inspection. When they are low to the ground you can detach the engines with relative ease compared to lifting them up from above the wing. This makes maintenance inspections quicker and thus cheaper.
During the walk around pilots can easily look into the engine intake, even grab the fan blades and turn them. You can have visual gauges like an oil level indicator at eye level.
In case of a fire hot burning oil doesn't drop down onto the wing and in case of an uncontained engine failure (e.g. turbine disk rupture) you at least have the wing in the way of the debris before it hits the cabin.


text:
What is the technique or procedure to disable/disengage the MCAS on Boeing 737 Max 8 & 9 Aircraft?
What is the technique or procedure to disable the MCAS and manually fly the Boeing 737 MAX 8/9 Aircraft?
To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?
UPDATE: The answer to this question explains how to disable/disengage the MCAS from pitching the nose down.
UPDATE: This video is the most concise answer to my question that I have found. It shows a crew in a full motion simulator like the Boeing 737 Max 8 that crashed in Ethiopia, then it shows the proper technique to disengage the MCAS and manually trim up in the same scenario.

While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be.

MCAS pushes nose down through use of Stab Trim when it detects excessive angle-of-attack from the AOA sensor. Unfortunately, when AOA provides erroneously high AoA, MCAS pushes the nose down unless it is deactivated. But crew must recognize this and flip those two Stab Trim switches off, then manually trim up. The video posted by @Korvinstarmast in the comments to his answer below show how this works in a full motion simulator.

So the conclusion seems to be, it's pretty darn simple to disable MCAS, disable Stab Trim, and manually trim up, done.

If the MCAS already put the trim too down, it could be too slow to trim up manually; unless you detect you need to avoid MCAS soon enough, I've been told (I don't know if the claim comes from an expert) that it's better to trim up (on the command column; this should disable MCAS for the time you input the command) and then disable stabilizers trim and proceed with manual trimming during the flight.

@ymb1 the person in the video you posted seems to be having much more difficulty turning the trim wheel- there is no audio and can't tell the context of the video, he might be faking it--his arm doesn't even look like he's trying. But good point, it does take time to trim up manually, but the video you posted seems like a bit of an exaggeration. Can any Max 8 pilots in here comment on how hard it is to turn the trim wheel?

MCAS doesn't have its own on/off switch
It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to be) encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model.  Its activation requires a number of preconditions, but we'll get to that in a moment.
A concise description of the system is found here.  The observation was a follow up to an observation that the MCAS isn't an anti stall feature like a stick shaker, but rather ...

Just about everything you have read in the media is wrong.  MCAS is not stall protection.  MCAS does not counter the additional thrust of the more powerful engines. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA longitudinal stability and stick force certification standards.  The easiest fix was to automatically apply a little nose down trim at high angles of attack.

A few things that should disable it (with caveats)

Lower the flaps. It is intended to work only if the flaps are up.

Turn the Stab Trim switches to OFF.  This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer).  This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight, though the Max trim wheels being of a smaller diameter raises a training and effectiveness problem1 that deserves its own question and answer.

The manual pitch trim procedure appears to be what a few crews did prior to the LionAir crash in October 2018.  It is unclear how many of the crews knew that it was MCAS that was malfunctioniing, versus any other trim or pitch anomaly.  The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source is the Preliminary Report from that accident).

... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed.  In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.
The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off.  The copilot flew the rest of the flight using manual controls and without autopilot.
That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. {snip} However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)

Enable autopilot.  It is supposed to only work if the autopilot is off.  But...CAVEAT

That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.
How do you know that you need to disable MCAS?
A related question is "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew is: how do you know what sub-system is giving you trouble?  The AD released in December of 2018 updated the Runaway Stabilizer procedure.  Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same (or similar) symptom may manifest during malfunctions with different causes.  (See the example above).  If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?
That concern, the relationship between different potential failure modes, gets to the heart of

What is in the pilots manuals?

What training the crews do, or do not, get on this sub-system?

The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off.  This includes high angle of bank flight in such conditions.

###CAVEAT
Take a look at that last bullet in the lower right of the figure.  It's only partially correct.  When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.

"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)

It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash.  They were airborne for about six  minutes before they were no longer able to overcome the horizontal stab position with their trim controls.  (Caveat: current as of early 2019.  When the final report comes out this may become a lot clearer, and may need revision).
There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here.  Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).
Why is this activating during takeoff/departure?
That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews.  There is a recent article in *Atlantic* that excerpts some NASA reports by aircrews in the US about some cases of unexpected / odd pitch behavior in this model aircraft.  As of mid 2019, the AoA signal (be it from signal error or a faulty AoA probe) has been identified as a part of the problem in this unexpected activation during takeoff and departure.

1 Note from @user40476, slightly edited:
If you are a bit late to switch off both CUT OFF switches, the deflected angle of the THS will create strong forces on the trim screw, more particularly if simultaneously you hold the yoke to maintain an acceptable pitch.
Critical Flight Safety Issue Follows
In these conditions, it becomes very hard to move the trim wheels manually. This is a major problem; the trim wheel diameter in the 737-MAX is smaller then the trim wheel of the previous models: 737-100, 737-200 etc. This smaller diameter reducing the efficiency of the pilot force on the wheel handle. (Less leverage for the same amount of pilot force applied).
Important Note: this answer is based on information available before the deep dive into the MCAS system that was required to recertify the 737 Max.  It will probably require revision when the final recertification process of the 737 Max is completed.

is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch?

you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim?  If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves?  Your graphic says "MCAS moves horizontal stabilizer trim".

Not my graphic, @Devil07, TheAirCurrent's -- :)  While I think you are right, I just need to recall which argument I was reading to link your thought to that.  Give me a few.  There is some interesting discussion on that here, based on the Lion Air accident.  See if it's helpful to you.  Beware, a few of the posters there are sharp, and a few are trolls.

@Devil07 This may help explain, or may not.

To answer your question, yes, as explained in this video

If you are a bit late to switch off both CUT OFF switches the deflected angle of the THS will create strong forces on the trim screw, more particularly if simultaneously you hold the yoke to maintain an acceptable pitch, IN THESE CONDITIONS IT BECOMES VERY HARD TO MOVE THE TRIM WHEELS MANUALLY, this is a major problem. The trim wheel diameter in the 737 max is much smaller then the trim wheel of the historical 737 -100, 737-200 etc, thus reducing the efficiency of the pilot force on the wheel handle.

@user40476 Thanks for concisely stating that.  I hope you concur with my slight editorial adjustment as I folded in your note.  Your point certainly squares with what I've been reading about, and while related to this question, I think for stack exchange purposes, that specific issue is its own question (and for Boeing, its own fix/problem to remedy) among others that these MCAS malfunctions have brought to light.

@korvinStarmast, thank you, I appreciate. Other fake news, it is said no trouble if you apply the trim runaway procedure! This is totally wrong because in the runaway down, pulling the column will block the trim; that’s not the case with MCAS.

You can disable the stabiliser trim which will prevent MCAS from making inputs to the aircraft.

This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim.

Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled.

that what I thought. Good answer and great picture!

From what I have read, the other answers explain the cutout procedure correctly.
Edit: The ongoing investigation appears to reveal that the aerodynamic forces on the stabilizer may become so high that it becomes hard or impossible to manually correct a nose-down trim. (This is not a B373 MAX issue but concerns earlier models as well.) The forces increase, unfortunately, with upward elevator position and high air speed, which is the situation in the final nose-dives. The remedy is described in the page user40476 linked to:

If the stabilizer put the plane in an extreme nose down position the pilot was advised to first pull the column to decrease the speed. He then had to push the column forward to lower the aerodynamic forces that blocked the jackscrew. Then the manual trim wheel could be turned a bit while the plane continued to dive and again increased its speed. The procedure had to be repeated several times: pull column to decrease speed; push column to decrease the aerodynamic force on the stabilizer and its jackscrew; trim manually; repeat. The technic was known as the rollercoaster maneuver.

Other sites (https://theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/, http://www.b737.org.uk/mcas.htm#mantrim) agree that the pilots may simply have been unable to operate the manual trim wheels because of the high aerodynamic forces.
Therefore the simulation in the video you link to where the co-pilot appears to turn the wheel with relative ease is probably incorrect in this respect.
[End edit]
It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:

[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1

This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.

1 The reason for this design decision being that providing the pilot a way to easily override

would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.

In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).
2 An article in the Seattle Times on March 17, 2019 presents that argument as well.

But what if it's a system's error and not pilot's? Shouldn't egineers have considered this case? Knowing that this MCAS considers input from just 1 AoA, that pilots could be not be aware of its existence, and that in situations can be hard to track the problem and apply the correct solution, even if someway easy (stab trim off)...

@ShinTakezou I agree with all your valid objections ;-). I just quoted possible rationales for the decisions as they were made.

I would say what you may read on the following website would also contribute to understand the difficulties that result from switching off the cutout switches:  https://www.moonofalabama.org/2019/05/boeing-737-max-crash-reveals-a-severe-problem-with-older-boeing-737-ngs.html

@user40476 Thank you for the pointer. It becomes clearer as the investigation advances how tragic these accidents were. The pilots may well have tried to do the right thing but the plane didn't let them, due to a flawed design.


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What is the data size of Hilton Software's WingX Pro 7 app and all of its databases?
What is the data size of the WingX Pro 7 installation files and databases including the app itself and the VFR Sectional, IFR Low Enroute, IFR High Enroute, and the full "All Charts(IFR)" databases?
I've noticed that the databases for different aviation apps have different sizes. What is the total storage for WingX Pro7 if you select the option to download all of the following databases:

List item
VFR Sections
IFR Low Enroute
IFR High Enroute
All Charts (IFR) - USA

My understanding is that WingX Pro7 uses higher compression than foreflight, leading to smaller database sizes, but I have been unable to find this information on their website, nor have their customer service responded to my email question.
UPDATE: I got my hands on a 16GB iPad Air and installed the latest version (July 2017) of WingX Pro 7 and downloaded all of the databases for the US including VFR and all IFR charts and plates for the Contiguous USA, plus terrain information. The only database I excluded was the High altitude IFR enroute charts. The total storage used by WingX Pro7 on the iPad is reported as 4.62GB.
2nd UPDATE: I finally got a response from HiltonSoftware on this question. They responded as follows:
Support  07/05/2017 at 2:25 PM
To: Devil07
Message body
WingX, fully loaded required 8GB
iOS may require at least 2GB free memory when all App are loaded
Space also has to be allowed for iOS

Support Team

The most you can get... Data sets are getting bigger all the time. Also depends how much distance you want to traverse, the father you go in one shot the more you need on hand.

We usually avoid questions like this because the answers can change so quickly. I'd contact Hilton directly to ask, they'll know how much space is required for various features and which hardware is compatible.

Its just that I've spent several hours researching this online and the answers are all hedged (i.e. get the most memory and fastest processor you can afford) general answers. I figured this would be the best forum to ask specific question about modern aviation navigation tools, and how they are actually being implemented in the cockpit. I'm not asking for recommendations on what brand is best.

Example: When I bought my iPad 2 years ago when I started flying I got the 16GB mini. It was more than enough at the time. Now that I fly farther and Foreflight has started to include more data like their new maps, and more airport info than they once did I push the limits of storage just keeping all the charts I need for the northeast. i.e. get the most you can now and it will last you for a while, eventually you will need to upgrade.

I'm a software engineer, not a pilot, but I don't need to know diddly about flying to answer this one. Get as much as you can afford. You quite literally cannot buy enough storage today to satisfy your needs a few years down the road, so save yourself some future money and hassle and just buy it now.

I think that everyone uses the same data from the AeroNav charting office of the FAA and it is no longer rasterized but is in vector form. Compression differences wouldn’t be an issue. ForeFlight says that the app plus all the downloads for the 48 states takes 6 GB of storage space.  (Not including the new Jeppesen charts.) If you start adding manuals, logbooks... Then you would need more. Plus, once you start using it you”ll probably want to add a few more aviation apps as well. 16GB is probably the bare minimum required. The next size up (32GB) is adequate. 128 GB  will last you for a while.

@JScarry that is interesting. Do you know when FAA changed to vector form?  My comment about Hilton using more compression was based on article I read online dated 2014, so its very likely it has changed since then. If you can track down some source to confirm current WingX total database size, please answer question and collect the bounty! :)

@Devil07 It turns out that they are not exactly vector files but GeoTiff format. Previously they were scans of the paper version and now they are completely digital. I don’t recall exactly but it was not more than 2 years ago. The neat thing is that you can zoom in really far the text is still readable.

This is a question about modern aircraft navigation equipment as provided for in the "help center". This is equivalent to asking a question about a panel mounted VFR GPS. After being out of aviation for a few years I missed the transition to digital, and this is a legitimate question by a pilot directed to other pilots about navigation equipment.

The app itself only takes up about 15-20MB, according to the Google Play or iTunes app store, where as ForeFlight takes up 226MB (almost a quarter of a gigabyte) and Avare is less than 2MB and free. After downloading a good handful of charts, Avare is about 250MB. If you are very tight on space and running Android, you could try Avare. Otherwise if you need WingX Pro 7 or are on Apple, 5-6GB is what you should have free if you plan to use all the charts. If you're buying a new tablet, try to go for a 32GB model to be safe and have plenty of room for other apps additionally, otherwise 16GB should also work.


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Guidance on becoming a pilot
Can anyone tell me which pilot training institute is best for getting CPL in India?
What other things are needed to be done along with CPL in order to become a co-pilot in an airliner?

Welcome to Aviation.SE! Specific recommendations like this are off topic here (a quick trip through the Tour should help you out. However, we have some other questions on what to look for in a flight school and what to look for in a flight instructor that should help you out.

@FreeMan the link you shared is for Italy.

That is very true, but if you look at Dave's answer you'll see nothing in there that's specific to Italy. Actually his answer is US-centric, based on his experience, but I don't think any of his points about what to look for are specific to any country.


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How much does the FAA private pilot's license checkride cost?
I can't find a strong number for this and I would like to know. I'm compiling a list of total cost for each part of getting a private license (training, study material, headset, medical exam, etc), but I can't find how much the checkride will cost.

Welcome to Av.SE, and best wishes as you embark on the adventure of learning to fly!

Also remember that while the FAA requirement to get a PPL is around 40 hours, most students average on 70 hours of instruction before getting their PPL. Just something to factor into your cost so you aren't surprised. Better to err on the high side than the low side.

Our two local DPEs are very busy so they can basically charge whatever they want. In April of 2019 they are charging $700 with a wait time until late June for exams.

There's no standard fee for the checkride because each Designated Pilot Examiner (DPE) is free to set his or her own price. I've seen numbers for a private checkride in the \$400-600 range depending on the location, but your instructor should know the local examiners and what they charge. Or just ask the DPE directly, of course. Bear in mind that you might have to pay extra if you have specific requirements like needing a checkride at short notice, on an awkward date, or at a particular location.
This question might be useful too, and see this one for the (unusual) option of using an FAA examiner instead of a DPE.

Anecdotally, I paid 450$ to a local DPEfor mine. My CFI said I might find an examiner through the FAA for less but you might have to schedule it further in advance and/or travel to the examiners airport.

@Thaumaturgic You can schedule with an Aviation Safety Inspector (same people who do ramp checks), but may not be available locally (for example, an ASI for the Milwaukee FSDO lives in Tennessee). I'm not sure what the cost is, I can't find any information on doing it on an FAA website.

@RonBeyer Using an ASI is free; they are paid to do it by the FAA. However, there aren't nearly enough to go around so are usually booked many months in advance, meaning you'll burn a lot of money staying proficient while you wait, so paying a DPE to get it done faster actually saves money in a sense.

I spoke to multiple DPEs in my area. You can find the list on faa.gov. One asked for USD 600. Another one asked for USD 400. The cost is not really the problem. The problem is finding a DPE who can administer the test at a reasonable date. The first DPE which my flight school recommended didn't have any dates available for 6 weeks. The second DPE didn't have any dates available for 3 weeks. Normally you don't want to wait that long when you have done checkride preparation with your CFI. You really want to get it done and over with and get your shiny new license :) Also as pondlife mentioned, you can use an FAA examiner who is free but they are booked 2-3 months in advance. Before you select a DPE, I would also recommend looking at this resource. Not all DPEs are the same. You don't want someone like this to take your checkride. On the broader question of costs, you might have missed out the following:

Landing Fee ( some airports charge a landing Fee  and over the course of your training you will do at least a 150-200 landings
Simulation equipment - I have a Saitek Yoke and Pedals and FSX on a PC. This will help you practice your lessons at home and will reduce your flight time and costs
Recording equipment - tripod, cables to connect to your phone etc. This will allow you to record your lessons so that you can replay them and reduce mistakes in future.
Insurance - the Flight School will have some insurance. But you might want to find out if this is enough for you and whether you want to purchase more
Calculator - unless you think you can master the manual calculator, you will need to purchase an electronic one, at least for your exam
Foggles for IFR Training required for private pilots
Cross Country software : Your plane may or may not be equipped with a GPS. I tried to use Avare on Android but was not very successful. Some people suggest foreflight but it is very expensive. This is an optional expense and you can get by without it at least for your private pilot training.
Last but not the least, once your CFI shows you how to checkout the plane, for all subsequent flights, make sure you get there 30 minutes before and checkout the plane by yourself so that you are not paying for CFI time.

"Some people suggest foreflight but it is very expensive." It's like $200 a year for a subscription. That's not very expensive.

Our local flight school maintains contact with an FAA examiner who was formerly associated with the school, and who charges a very reasonable $150 last I knew. Doesn't live that far away, so his drive up to the airport where the school isn't a big cost driver in the examiner's fee.


text:
How do I calculate density altitude by hand?
I've seen the following equation for calculating density altitude on a dozen websites:
[ (OAT - ISA) * 120 ] + Pressure Altitude.

But it was never very clear what the units of measurement are supposed to be.
Question 1: Are OAT and ISA in Celsius?
Question 2:
So for example, if current true altitude is 6000 feet above sea level then:
ISA = 15 + (-2C * 6) because for each 1000 feet we lose 2 Celsius. Does this mean to make this calculation I just substitute field elevation for true altitude? So ISA is equal to 3.
If the outside air temp is 10C, then we have 10 - 3 = 7 multiplied by 120 = 840.
Question 3:
Pressure altitude confuses me a little bit, I read that it is the altimeter reading when the Kollsman window is set to 29.92" hg. So let's say I'm sitting in my airplane on the runway, I set my altimeter to 29.92 and it reads 6200 feet. In this case my final equation would be: 840 + 6200 = 7040 feet density altitude.
What happens if I don't have access to my plane's barometer, is there another way to calculate pressure altitude?
Question 4:
How does this all change if I'm trying to figure out density altitude while flying the plane? OAT is the air around the plane. What about ISA? Do I calculate pressure altitude by resetting my Kollsman window in the middle of the flight to 29.92" hg? Is it normal to try to figure out density altitude while up in the air?
Question 5:
Does this entire equation change or become meaningless above a certain altitude threshold, such as 10,000 feet above sea level?

Density altitude is a value that directly correlates aircraft performance to atmospheric conditions.

Density altitude is pressure altitude corrected for non-standard temperature.

Almost all temperatures in aviation is Celsius including this formula.

You will notice that all aircraft performance calculations require pressure altitude and temperature and in effect is calculating density altitude.
To calculate pressure altitude you will calculate
$$ \left( 29.92 - \frac{\mathrm{current~altimeter~setting}}{\mathrm{inHg}} \right) \cdot 1000 \, \mathrm{ft} + \mathrm{current~field~elevation}$$
For example if the current altimeter is $30.12 \, \mathrm{inHg}$ and you are $ 6,400 \, \mathrm{ft} $ MSL, the pressure altitude is
$$ \left( 29.92 - 30.12 \right) \cdot 1000 \, \mathrm{ft} + 6,400 \, \mathrm{ft} = 6,200 \, \mathrm{ft}$$
pressure altitude.

Your formula to calculate density altitude is correct but is never required to be known to calculate aircraft performance as the "calculation" is being done in the background of the performance charts.
To calculate density altitude, you must first calculate pressure altitude and then apply it to your formula.
If you are at 10°C at $6400 \, \mathrm{ft}$ MSL with an altimeter setting of $30.12 \, \mathrm{inHg}$:
$$(10-2.6) \cdot 120 \, \mathrm{ft} + 6,200 \, \mathrm{ft} = 7,088 \, \mathrm{ft}$$ density altitude

I have never calculated density altitude once I got into the air.  I have calculated density altitude while determining if its a good idea to takeoff.  In higher elevation airports on a warm day, the airplane may not get out of ground effect due to the density altitude.
The formula for density altitude above is correct for any altitude, albeit a more simplistic formula.  Humidity also has an effect of density altitude and there are formulas that take humidity into account when calculating density altitude.

I see! So to calculate ISA you also use the pressure altitude, ie
15 + [ (6200 / 1000) * -2 ] ? As for pressure altitude, I think I got confused because I thought pressure altitude was always the plane's altimeter reading when purposely set to 29.92

Sorry one more question - when you say "current_altimeter_setting", is that what you've set in the Kollsman window? Does that mean it's the value you would get from the airport weather report/radio?

The pressure altitude is read from the altimeter when you set the altimeter to 29.92.  You can use the formula above to calculate pressure altitude knowing field elevation and the altimeter setting.  The altimeter setting is given by the airport weather service.  (ATIS, AWOS or ASOS).


text:
What is the FAA definition of "takeoff" and "landing"?
This question is in the context of commercial flights with passengers.
What is the definition of "takeoff" and "landing" phases? Does it include taxi? Is it from lift-off for takeoff? and touchdown for landing?
A link to the official document would be much appreciated.

Depends on the context. Simply being on a runway with the intent or clearance to takeoff could be part of takeoff. when it ends could be wheels off the ground, out of ground effect, accelerated to Vx, clear of obstacles, Vy, leaving pattern altitude or company rules about sterile cockpit could set a specific altitude. Context matters.

According to the FAA FAA Airplane Flying Handbook, there are three components that make a takeoff:

Takeoff roll (ground roll) is the portion of the takeoff
  procedure during which the airplane is accelerated
  from a standstill to an airspeed that provides sufficient
  lift for it to become airborne.
Lift-off is when the wings are lifting the weight of the
  airplane off the surface. In most airplanes, this is the
  result of the pilot rotating the nose up to increase the
  angle of attack (AOA).
The initial climb begins when the airplane leaves the
  surface and a climb pitch attitude has been established.
  Normally, it is considered complete when the airplane
  has reached a safe maneuvering altitude or an en route
  climb has been established.

So to have a take-off you must have these three components.  That would include a touch and go as well takeoffs from a full stop.
As far as an official definition of landings I didn't see anything official in the FAA documentation but I would say that the airplane touching the ground would count as a landing.

The Airplane Flying Handbook is not regulatory.

Normally, definitions are contained in 14 CFR 1.  That part does not define takeoff nor does it define landing.  Since they are not defined there, and to my knowledge anywhere in the regulations, a generalized definition of takeoff or landing.
Therefore, the common definition (or really usage) of takeoff or landing would apply, at least in legal matters.
One should be careful deriving definitions from advisory material.
It is noteworthy that for different aircraft, takeoff may mean different things.  For example, on a 787, when the mains leave the surface of the runway, we might consider that takeoff.  However, a R22 helicopter, hovering on a ramp, or taxiing to a runway may not be considered as having taken off, even though it is not in contact with the surface or a surface structure.
Takeoff has different nuances, and mechanics, for different aircraft, and even different operations.  There is no one regulatory definition, although some aspects of a definition may be inferred from some regulations. Advisory material do not constitute regulations.


text:
Does a portable ILS ground station exist?
Does anyone know of a good example of a portable ILS ground station?  I mean, it can be used for temporary airfields.

I don’t think there is such a thing. That’s why the military developed TACAN which can be used for temporary sites or small sites like aircraft carriers.

Related: https://aviation.stackexchange.com/a/57267/35166

@Gerry please don’t post answers in the comments, as there’s no quality control (I.e. your comment can not be downvoted).

How portable is portable? Is being able to ship it around the world portable enough? How about an aircraft carrier?

NASA did some research on it in 1986 and published this paper. Thales was contracted to build one that the US Army has ordered and it made it to prototype phase. Eventually it was tested for a while and appears to have made it to production and the military ordered more.
It goes under the name Deployable – Instrument Landing Systems  (D-ILS). here is a short two page brochure on the unit which has some good pictures of the whole thing.
It looks like the units cost around $4.68 million each.

It makes sense the military would have a field deployable ILS system.

Note that "deployable" means "can fit in a set of standard shipping containers and only takes five days to set up", not "can drop it at the end of the runway and start landing planes".


text:
Can a pilot use legal psychoactive cannabinoids?
How do the FAA and airline companies treat pilots using substances like Delta-8-Tetrahydrocannabinol on their down time? Like CBD, Delta-8-THC is a legal derivative from marijuana.

Taxiway delta 8 is ok though.

Related

Delta-8-Tetrahydrocannabinol (Delta-8-THC, Δ8-THC) is a cannabinoid found in the cannabis plant. It has psychoactive properties; the effects are less potent but comparable.
Delta 8 THC metabolites trigger many drug tests looking for Delta 9 THC metabolites. These compounds are chemically similar.
Whoever needs to pass a drug test (pilots included) should not take them.

I made a mistake of  saying it had no psychoactive properties but I corrected it before there was an answer.

Yep, if you want to fly, stick to alcohol.  In Canada, pot is legal now, but Transport Canada policy is zero tolerance and will still ground you if you have any THC or THC derivatives in your system.

There was a fatal train crash in London in 1991 when the driver failed to stop in time at a terminal station and the report said the driver may have been using cannabis. An expert witness said that this drug impairs time and distance estimation skills, and, paradoxically, causes the the user to feel that they are improved.


text:
Would an airliner fly more efficiently on a fatter wing at a low altitude?
Flying at 1000 ft, would giving an airliner wing more girth but not length, allow the plane to fly more efficiently than it normally would at that height?
Yes, airliners fly best around 35,000 ft, but if it was restricted to fly at 1000 ft., what would be the best shape wing for it?

With that limit, better build it as a ground effect vehicle for intercontinental traffic across oceans.

Thermal efficiency reduces

@PeterKämpf ...which could also lead us to the question: what is the minimal cruise altitude below which flying in ground effect is more efficient?

@Gypaets: Yes, that is a very good question. I had once the task to do preliminary layout for a ground effect vehicle and used some optimization algorithm to do so. It resulted in a typical airliner, flying at 40 kft. I did not put in a height limit and the crossover point was at low altitude, but I cannot remember the exact figure.

Silly comments aside, "fat" wings, ones with greater camber, were all the rage 100 years ago when designers found they could eliminate draggy external bracing by building cantilever wings.  Hugo Junkers was one of these pioneers.
Wind tunnel testing also showed increased camber not only increased lifting efficiency, but also, combined with large, rounded leading edges, produced very benign stalling characteristics.  One result of this research, the Fokker D.VII., so terrified opposing pilots that it was specifically banned after the end of the "Great War".
Heavily cambered wing design such as the Davis wing lived on until aircraft began pushing the limits of the "sound barrier".  Aircraft fly higher to get more speed with the same amount of drag, and this works great until the wing approaches its critical Mach number.  Above this, drag greatly increases, ending the reign of the "fat wing".
Modern "super critical" wings, literally "surf" the slower bottom airflow, leaving the top of the wing as flat as possible.  This is what airliners use at 35,000 feet.
But at 1000 feet, where airspeed is limited to 250 knots, the "fat wing" makes a comeback by deploying slats and some flap.  For a given wing area and Angle of Attack, the "thin wing" is only more efficient when Critical Mach Number is approached.
This is why many airliners have a never exceed speed for lower altitudes, and a Mach limit for higher altitudes.

Please - it's Fokker D.VII.


text:
When is flotation gear legally required for pilots and passengers on a non-commercial flight in a small airplane over water?
If I'm renting an airplane to fly myself and some friends over water beyond gliding distance, but within 50 nautical miles from the nearest shore, am I legally required to have any flotation devices whatsoever?
According to 91.205 if you are flying for hire over water you must have life preservers for each passenger. However, does renting an airplane for recreational use trigger this requirement? In other words does renting and aircraft from an FBO qualify as operation for hire for purposes of section 91.205?

I did multiple text searches in the CFRs and can't find any other sections that refer to over water requirements. A related question, (not sure if I should ask it separately) is whether renting an aircraft from an FBO constitutes a flight for hire. If I rent a Cessna 172 and fly myself over water, is that considered an aircraft being operated for hire since it is being rented?

Apparently, the answer is never.
After several days of searching, I was unable to find anything in the FARs that requires the pilot of a small aircraft that is NOT being flown "for-hire", to have flotation devices on board, for a flight over water, regardless of the distance from shore.
Section 91.205 requires flotation devices, but only for flights "for hire" over water. So this section doesn't apply to this question.
Then, there is section 91.509(12) which requires flotation and other survival gear for flights 50 nm, or 30 min out from shore, but this entire section is under Subpart F - Large and Turbine-Powered Muti-engine Airplanes and Factional Ownership Program Aircraft. So this entire section doesn't apply to this question either.

Common sense says: unless ou are flying exclusively over a desert you do need safety floation gear. In addition: even if you are a great swimmer you can find dificult to keep afloat after a hard landing broken some of your limbs

I'm not saying its smart, just saying it isn't illegal when its done in a small aircraft that is not being flown "for-hire."

@jean The question is when is it legally required, which this answers perfectly.

FAR 91.205 contains minimum equipment requirements, and flotation devices are only required during "for-hire" operations.  That doesn't necessarily override common sense, though...

91.509, you're right, is for large (>12,500GW), turbine, and fractional ownership aircraft, so it depends on what type of plane is being rented, so not necessarily only "for hire" operations.

True enough, but I don't think the question is coming from a professional pilot...

@ronbeyer I just noticed that I missed the fact that 91.509 is for large aircraft (subpart F - large and turbine powered multiengine airplanes). I missed that. Now, I'm back to where I started, I can't find anything that says a small plane needs to have flotation devices if the flight is not for hire.

Correct.  "Not for hire" = no required flotation devices.  "Common sense" = "bring flotation devices if you're not happy swimming to shore."  Give the Coast Guard a sporting chance...